Maskasky U.S. Pat. No. 4,435,501, hereinafter referred to as Maskasky I, observed that the radiation sensitivity of high bromide {111} tabular grains can be enhanced when a site director, such as iodide ion, an aminoazaindene, or a selected spectral sensitizing dye, is adsorbed to the surfaces of the {111} tabular grains to restrict silver salt epitaxial deposition to selected sites, typically the edges and/or corners, of the tabular grains. Maskasky I in Examples 2 and 3 demonstrated that silver chloride grown epitaxially outwardly from the edges (Emulsion 2C) or outwardly from the corners (Emulsion 3B) of silver bromide {111} tabular grains produced a much higher photographic speed than silver chloride epitaxy grown randomly over the major faces of similar host tabular grains (Emulsion 2B, Emulsion 3A). By comparing Tables II (column 65) and III (column 66) it is evident that Maskasky I observed a larger speed increase when the silver chloride epitaxy was deposited as protrusions from the corners of the host tabular grains than as protrusions from the edges of the tabular grains. Maskasky I states that, in general, larger increases in sensitivity are realized as the epitaxial coverage of the major crystal faces decreases (column 21, lines 11 to 13 inclusive). Although the inclusion of minor amounts of iodide in the epitaxy is specifically contemplated (column 23, line 38), Maskasky I states that it is generally preferred as a matter of convenience that the silver salt epitaxy exhibit a higher solubility than the silver halide of the host tabular grain (column 24, lines 10 to 12 inclusive).
Maskasky U.S. Pat. No. 4,471,050, hereinafter referred to as Maskasky II, discloses that nonisomorphic silver salt can be selectively deposited on the edges of silver halide host grains without relying on a supplemental site director. The nonisomorphic silver salts include silver thiocyanate, .beta.-phase silver iodide (which exhibits a hexagonal wurtzite crystal structure), .gamma.-phase silver iodide (which exhibits a zinc blende crystal structure), silver phosphates (including meta- and pyro-phosphates) and silver carbonate. None of these nonisomorphic silver salts exhibits a face centered cubic crystal lattice structure of the type found in photographic silver halide--i.e., an isomorphoric face centered cubic rock salt crystal structure. In fact, speed enhancements produced by nonisomorphic silver salt epitaxy have been much smaller than those obtained by comparable isomorphic silver salt epitaxial sensitizations.